Difference SIP and PJSIP installation

What is the difference entry SIP channel and PJSIP channel.

As I checked in the Asterisk installation chan_pjsip.so is a module from the default installation, then, my doubt is, why has this website https://www.pjsip.org/ if pjsip is automatic included in the Asterisk default installation?

I need to install the PJSIP in separate or can I use the chan_pjsip.so installed with asterisk by default?

PJSIP is a separate project, not created or maintained by the Asterisk team. It’s used in many projects, including Asterisk. The chan_sip module uses our own SIP stack and is no longer actively maintained. As of Asterisk 16 PJSIP is automatically downloaded and chan_pjsip built. There’s no extra work required to build it in. In the case of Asterisk 13 you need to pass an option to configure[1] to do so.

[1] https://blogs.asterisk.org/2016/03/16/asterisk-13-8-0-now-easier-pjsip-install-method/

Interesting. thank you